Sintered metal carbides, including cemented carbides in which carbide particles such as tungsten carbide, titanium carbide, tantalum carbide or niobium carbide particles are cemented together with a binder metal such as cobalt, iron, nickel or an alloy of one or more of these elements, are widely used as cutting tools, drilling tools and wear parts.
Reclamation of the components from scrap has been a problem for the carbide industry since 1926 when Schroter first used "members of the iron group" (iron, nickel and cobalt) as binders for tungsten carbide. The members of this group wet the grains of tungsten carbide, making it very difficult to remove them from between the grains during reclaiming. However, because of the increasing prices of the component metals, notably tungsten and cobalt, it is becoming increasingly attractive economically to reclaim one or more of the components from these carbides.
Various processes are known to disintegrate the cemented carbide into a powder mixture which, with or without further processing, can then be recycled to make more carbide. Such processes have included alloying with zinc, acid leaching and fusion with nitrates.
In U.S. Pat. No. 3,595,484, the binder metal is dissolved in molten zinc to break the bond with the carbide, followed by distillation of the zinc to leave behind separated particles of carbide and binder metal, which can then be reprocessed.
Other disintegration methods are described in: U.S. Pat. No. 2,848,313 (increased internal stresses caused by the formation of a hydrated salt); U.S. Pat. No. 3,184,169 (pneumatic pulverizer); U.S. Pat. No. 3,438,730 (milling in the presence of phosphoric acid and free oxygen); and U.S. Pat. No. 3,635,674 (subjection to ammonia or ammine and free oxygen under pressure).
In U.S. Pat. No. 3,953,194, the cemented carbide is subjected to catastrophic oxidation, converting it to a friable, easily-crushed mass, crushed to powder, reduced to metal, and carburized to obtain a mixture of tungsten carbide and cobalt powders for reprocessing.
Other processes are available to separate the component metals or oxides after oxidation or reduction. For example, U.S. Pat. No. 2,704,240 calls for dissolving the cobalt values in an inorganic acid such as sulfuric or hydrochloric followed by recovery of the cobalt from solution. U.S. Pat. No. 3,887,680 calls for separation by digestion of the tungsten values in an alkali metal hydroxide in order to form an aqueous soluble alkali metal tungstate.